Holy uprising, oddly trendy, broke out (7)
I believe the answer is:
erupted
'broke out' is the definition.
('erupt' can be a synonym of 'break out')
'holy uprising oddly trendy' is the wordplay.
'holy' becomes 'pure' (I can't explain this - if you can you should believe this answer much more).
'uprising' says the letters should be written backwards.
'oddly' means one should take alternating letters.
The alternating letters of 'trendy' are 'ted'.
'pure' back-to-front is 'erup'.
'erup'+'ted'='ERUPTED'
Can you help me to learn more?
(Other definitions for erupted that I've seen before include "Broke out suddenly" , "Blew up or out as volcano may" , "Broke out (like Etna)" , "Blew up like volcano" .)